During transmission of Ethernet frame data on a SDH transmission network, when transmitted data volume at the sending end excesses possible receiving data volume at the receiving end, and has not been aware, then huge data will be lost. Therefore, it is necessary to have a data flow control to guarantee that data transmission is normal and reliable.
At present, in an Ethernet data packet, there is only one kind of control frame defined, it is flow control frame (such as PAUSE frame). The PAUSE frame controls data flow through a time parameter. When the sending data volume is greater than the possible receiving data volume, the receiving end will send a PAUSE frame to the sending end and the sending end will pause data sending. When the receiving end can normally receive data, the receiving end will again send a PAUSE frame to allow the sending end to send data normally. When sending a PAUSE frame, the PAUSE frame is directly inserted to a data queue, but cannot interrupt the data being sending, so the PAUSE frame must wait until the current data sending is finished then the PAUSE will be sent. In the receiving port, when a PAUSE frame is received, first it is detected, interpreted and analyzed. After the PAUSE frame has been identified and recognized, flow control operations will be made accordingly, such as setting flow control data, flow control time etc.
Although, the flow control with PAUSE frame is a safely manner for data packet transmission, but because of transmission delay, the flow control is effective only within a limited scope, in general it is several hundred meters. Nevertheless, at present many SDH facilities needs to transfer Ethernet data with distance far more then several ten kilometers, or even several hundred kilometers. Therefore, flow control is an important and unsolved problem.